Think you can do this with digikam but I need to double check

Edit: straight from their docs. Also they state "Tags are stored in a database for fast access, and, the applied tags are written into IPTC data fields of the image (at least for JPEG). So you can use your tags with other programs or, in case of loss of that data in the database, the tags will be re-imported when the image is noticed by digiKam." I would do a few picture first as a test to see if it works to your liking

The only option I would see to add tags to files without using some sort of picture management program that supports tags or an SQL database would be to just add raw text data to images. The image still works as eg. "IEND.B`."/"0x49454E44AE426082" marks the end of png data. Everything afterwards is ignored. You can test that by writing some text into a file (echo "Hello World!" > test.txt) and cat'ing the image + the text, eg. cat image.png test.txt > new_image.png. new_image.png will still contain the original image. Note that editing/exporting in editors will probably delete that data. Still, it could be used to add tags in eg. Json/CSV, and also extract them. That would require some work in coding, but work on any filesystem and system without additional files.

Digikam is powerful and can do this no problem. Digikam can store in its own database and (if you choose) at the same time in the image itself in case your digikam database ever gets corrupted.

I recommend embedding something like a checksum in the filenames to prevent duplicates.

XnView has tag and filter feature.

I use XnView MP to view and organize my photos.

Fairly certain that you can encode tags into the photo's metadata.